Halogenated hydrocarbons are aliphatic or aromatic hydrocarbon compounds composed of hydrogen and carbon with at least one hydrogen substituted by a halogen atom (Cl, Br, or F). Halogenated hydrocarbons are used for many purposes, such as solvents, degreasers, pesticides, and dry cleaning agents, and are one of the largest and most recalcitrant groups of contaminants found in groundwater. As a subgroup, the chlorinated aliphatic hydrocarbons, consisting of such compounds as methylene chloride, chloroform, carbon tetrachloride, tetrachloroethene (PCE), and trichloroethene (TCE), are commonly referred to as the "chlorinated solvents." As used herein, the term "chlorinated solvents" shall refer to chlorinated aliphatic hydrocarbons.
As a result of their widespread use, the chlorinated solvents are among the most prevalent groundwater contaminants. In fact, a 1984 survey of water supplies in the United States found that PCE, TCE, and the three isomers of dichloroethene (DCE) were the five most frequent contaminants found in groundwater other than the trihalomethanes.
Contamination of groundwater by chlorinated solvents is an environmental concern because chlorinated solvents have known carcinogenic and toxic effects. For example, carbon tetrachloride is a systematic poison of the nervous system, the intestinal tract, the liver, and the kidneys. Vinyl chloride, which is used in the manufacture of polyvinylchloride (PVC) and is a degradation product of chlorinated ethenes (PCE, TCE, and DCE), is a known carcinogen, and also can affect the nervous system, the respiratory system, the liver, the blood, and the lymph system.
Chlorinated solvents are among a group of heavier-than-water hydrocarbons that often are found in separate phase mixtures in the subsurface called dense nonaqueous-phase liquids ("DNAPLs"). DNAPLs are visible, denser-than-water, separate oily phase materials in the subsurface whose migration is governed by gravity, buoyancy, and capillary forces. When in contact with groundwater, soluble constituents in the DNAPL (such as chlorinated solvents) partition into the water phase to create a dissolved contaminant plume. DNAPL thus can serve as a long-term, continuing source of contamination as the soluble constituents slowly dissolve into moving groundwater.
DNAPLs comprised of chlorinated solvents present a formidable remediation challenge for four reasons: (1) the density of DNAPLs causes the contaminated zone to spread deep below the water table; (2) chlorinated solvents have physical properties that allow movement through very small fractures in the soil (&lt;20 microns) and downward penetration to great distances, even through some clay strata; (3) strong capillary forces make the removal of individual DNAPL trapped in soil pores very difficult or impossible; and, (4) chlorinated solvents are not readily biodegradable under natural conditions and can persist for long periods of time in the subsurface.
Several techniques have been applied to the remediation of sites contaminated by chlorinated solvents; however, most have proven to be costly and inefficient.